Answer:
Bartolome de las Casas witnessed the brutal treatment that the spanish conquistadors gave to the Native Americans who lived in Hispaniola and Cuba.
They natives were killed frequently in conflicts, became enslaved, and many died while enduring forced labor.
This horrified de las Casas, because he believed that the natives were equally rational and humane as the Spaniards. The only thing that made them different was that the Natives were not christians.
Bartolome de las Casas argued for the end of the enslavement of native americans, and he succeeded, in a way, because slavery was replaced with a system known as "Encomienda". Under the Encomienda, natives are christianized, are not formally slaves, but more like serfs. This system was more or less a reenactment of medieval feudalism, but in the Spanish Americas.
Bartome de las Casas did not recommend Encomienda either, his views were probably misinterpreted.
